The invention relates to a driver circuit for a laser diode and, in particular, to a laser driver circuit for a laser diode with improved noise level.
Optical communication systems commonly employ semiconductor lasers or other optical emitters for transmitting light signals indicative of a low data value and a high data value, as represented by different light intensity levels. Semiconductor lasers, such as edge emitting lasers (Distributed Feedback (DFB) lasers and Fabry-Perot lasers) and vertical cavity surface emitting lasers (VCSEL), are well known. Semiconductor lasers or optical emitters may be used to form a fiber optics transmitter or an optical transceiver.
A semiconductor laser, also called a laser diode, requires a bias current above a threshold level applied to the laser to turn the laser on so that lasing can occur. Once the laser is turned on, that is, the bias current is above the threshold level, the laser can transmit data signals (high/low data values) by receiving a data-dependent modulation current which operates to modify the optical power emitted by the laser diode. The two levels (high or low) of the data pattern are usually represented by a large emitted optical power or a small emitted optical power. A laser driver, typically implemented as an integrated circuit, is used to apply the desired bias current and provide the data-dependent modulation current.
Both the laser bias current and the laser modulation current are monitored and adjusted over time to compensate for variations in the average output power level due to laser aging, temperature variations, and other factors. FIG. 1 is a circuit diagram illustrating a conventional laser driver including a feedback control for regulating the laser diode modulation current. In laser driver 10, the input data (IN and INb) is coupled to a differential circuit 20 as the input stage of the laser driver. An output circuit 26 formed by transistors Qout1 and Qout2 configured as a differential pair provides an output signal (OUT and OUTb). In the present description, the output signal is an output modulation current signal coupled to drive the laser diode as a load. The differential pair of transistors Qout1 and Qout2 is biased by a current source formed by a resistor RMOD. The voltage VRMOD across the resistor RMOD is regulated to the reference voltage VMOD and the modulation current IMOD is given as VMOD/RMOD. In most applications, the resistor RMOD is an integrated on-chip resistor. In that case, the required modulation current IMOD is obtained by selecting a reference voltage VMOD such that the desired modulation current is realized using the fixed value resistor.
To monitor and control the value of the modulation current, a feedback loop is formed where a feedback voltage VFB is taken from the current source resistor RMOD and coupled to an operational amplifier 16. The operational amplifier senses the difference between the feedback voltage VFB and the reference voltage VMOD setting the desired modulation current value. The operational amplifier 16 generates an output signal 18 which is coupled as the positive supply voltage VR for the differential circuit 20. In other words, the operational amplifier 16 adjusts the positive supply voltage VR of the differential circuit 20 to adjust the modulation current IMOD.
However, because resistor RMOD is connected to the emitter terminals of transistors Qout1 and Qout2 which are moving in accordance with the input data pattern, the voltage VRMOD across the resistor RMOD is not a stable or constant value. Instead, the voltage VRMOD has a data dependent variation where the data pattern is usually a bit pattern at a certain data rate. Because the voltage VRMOD is taken as the feedback voltage VFB, the data pattern of the input data influence the feedback loop. As a result, the modulation current IMOD provided at the output terminal 27 is not regulated to a constant value but rather exhibits a data pattern dependency. The data pattern dependency is particularly problematic for low output data values.